A satellite communication system can divide a service area into multiple cells, and configure a satellite to direct a spot beam towards the center of each cell. The directivity of the spot beam will be highest at the center of the cell and roll-off towards the distal regions of the cell. The satellite communication system can apply “frequency reuse” in which two or more cells use the same frequency band, provided such cells are adequately distant from one another. An increase in the number of cells can enable increase in reuse of frequency bands, which can increase total capacity to the service area.
However, increasing the number of cells over a fixed service area reduces their size and, therefore, can require a corresponding increase in antenna directivity, i.e., a narrowing of spot beam width to match the smaller cell area, with acceptably uniform coverage, and without spanning into adjacent cells. The latter can waste power, degrade received signal quality and, with respect to frequency reuse, can require an increase in the minimum separation of reuse cells. In typical satellite systems the directivity at the edge of the cell is 3-6 dB lower than the directivity at the center of the cell.
Increasing the satellite antenna transmit/receive directivity can carry a range of technical problems. One problem is that increasing antenna directivity necessarily decreases tolerance for absolute pointing error. As the beamwidth decreases, a fixed absolute pointing error becomes a larger fraction of the beamwidth. This means that users closer to the cell boundary are necessarily close to a steep fall-off of the antenna directivity and, hence, susceptibility to pointing error increases.
When adjacent spot beams are created by different antennas (as for example in the case of high throughput satellites) it is possible for the different antennas to mispoint in such a way that some portions of the coverage area fall outside an acceptable directivity level of any of the spot beams. Such portions may have a significantly degraded performance.
One potential solution is to apply structural modifications to the satellite, or more robust control of platform attitude and orientation, or both, so as to attain a desired pointing accuracy and stability. However, for some applications the level of such techniques needed to achieve the desired directivity may be unacceptable.
Another potential solution can be dynamic reassignment of cell boundaries to match current spot beam pointing. This may be referred to as “moving cells.” However, although this solution may be feasible for some applications, there can be technical issues, such as complexity in handover between the “moving cells,” that can render this solution not feasible. Moreover, in the case of multiple antenna platforms, some areas may not be covered by any beam and hence handover is not an option.
Accordingly, for reasons such the examples addressed above, there exists a technical need in satellite and other wireless communication for low complexity, high directivity multi-beam communication, with high spectral reuse and strong tolerance to pointing error.